Despite strong epidemiological data associating tobacco smoke to lung cancer, the mechanism of carcinogenesis by tobacco chemicals is not well understood. Among those exposed to tobacco smoke, only a certain portion develop lung cancer suggesting the involvement of genetic factor(s) in the susceptible population. A difference in DNA repair ability has been suggested for the explanation of the susceptibility. Of the various DNA repair pathways, a relationship between nucleotide excision repair and tobacco carcinogens (e.g. benzo[a]pyrene) has been well documented. However, DNA double strand break (DSB) repair, another important repair pathway, has rarely been studied with tobacco chemicals in mammalian systems. Since a recent study revealed a positive association between lung cancer and the reduced activity of a protein involved in DNA DSB repair, this application is initiated to confirm and extend the study on the role of DNA DSB repair in tobacco carcinogenesis. DNA DSB is the most destructive form of DNA damage and can lead to cell death, mutation and transformation if not repaired or mis-repaired. An efficient way to study the role of DNA DSB repair is to use DNA DSB repair deficient mutant cell lines exposed to tobacco chemicals. Specific Aim 1 addresses: Various rodent and human DNA DSB repair deficient cell lines will be treated with several tobacco carcinogens and cigarette smoke condensate (CSC), and comparisons will be made to wild type cells with regard to cell survival and mutation frequency. This is to test the hypothesis "Mammalian cells utilize DNA DSB repair mechanism or protein(s) associated with DNA DSB repair in the recovery process of DNA damage induced by tobacco carcinogens". Further molecular studies are suggested based on the cell line studies. These studies have the potential to identify a new tobacco sensitive sub-population. The experiments in Specific Aim 2 are proposed based on our recent finding that lung cancer sensitive BALB/c mice showed a mild DNA DSB repair defect. Specific Aim 2 puts forward the studies on the cell viability, DNA DSB formation and chromosome rearrangements in alveolar macrophage (AM), lymphocytes and lung epithelial cells from BALB/c and C57BL/6 (control) mice exposed to tobacco carcinogens. Two hypotheses will be tested in this aim: (1) Cells from BALB/c mice show a higher level of chromosome misrejoining which is detected by a sensitive premature chromosome condensation (PCC) assay in interphase chromosomes combined with/without fluorescence in situ hybridization (FISH)." (2) There is a good correlation between the sensitivity data with lymphocytes and those with AM and/or lung epithelial cells. The proposed experiments not only provide new mechanistic insight on the initial step for tobacco carcinogenesis, but also help furnish sensitive diagnostic tools to identify lung cancer susceptible individuals.